A.Master electrician Sean Kenney responds: It is certainly possible that the breakers were defective. I have noticed that quality control for electrical equipment has gone down considerably in the past 20 years or so. I would be especially suspicious if the breakers were purchased at a home center or hardware store.
If the panel was replaced to try to solve a problem with tripping circuit breakers or fuses, a contributing factor may be problems with the house wiring. If the house had an overloaded circuit, and the homeowner deliberately installed oversized fuses to keep them from burning out, then correctly sized breakers in a new panel will be easier to...
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